Ferrets were domesitcated due to the fact they are natural hunters, same as dogs. They made catching rabbits to put on the plate much easier. Seeing as there was plenty of rabbit, may as well feed the ferrets on it too. Convenient and easy. No proof whatsoever that ferrets lived soley on rabbits when they were originally wild.
CAN I JUST POINT OUT THAT PRE-MIXI IN THE EARLY 1950's RABBITS WERE A MAJOR PROBLEM WITH A PRICE ON THEIR SCUTT,BUT FERRETS WERE FED ON AND SURVIVED ON A BREAD AND MILK DIET
As obligate carnivores, bread, soya milk and veg has no nutritional value to ferrets at all